Within a fibre optic cable such as that used in optical communications, visible light signals travel along the cable bounded by multiple glass layers with different refractive index properties; through a process known as total internal reflection these layers trap the light in the fibre core so it cannot escape until it reaches the end of the cable where it can be collected/analysed.
It is well known to include fibre optic cables in an optical sensing network by interfacing them to optical sensors to detect, for example, stress/strain, temperature, humidity, pressure or other properties. Many of these optical sensors use components called fibre Bragg gratings (FBGs). A Bragg grating is a periodic refractive index variation that will reflect light only at a precise colour or wavelength—known as the Bragg wavelength. When these structures are present in optical fibres they are known as FBGs and the precise wavelength of the back reflected light will depend on the environment local to the FBG. This concept is now routinely exploited to create distributed sensing networks for many applications.
A key component of such systems is the interrogator. This is the device that measures the wavelength shift and produces the calibrated signal that provides the measure of temperature/stress/strain or other property of the system under test. This is where the main cost of a system lies. The interrogator system itself is a precision optical analyser comprising fibre connectors, mirrors, optics and detectors that is bulky (often shoe box sized), expensive to build, can be easily damaged and is a major barrier to mass market uptake.
It is known to form waveguides within bulk material, which operate in a similar fashion to optical fibres. The waveguides are formed by using ultrashort pulse laser inscription to modify the refractive index of the material along a path through the material. One example of a method for forming waveguides in bulk material is described in WO 2008/155548, which is hereby incorporated by reference.
It has also been suggested to form Bragg gratings in such waveguides by varying the refractive index properties along the path. For example, single scan ultrashort pulse laser inscription using either low repetition rate systems where the period of grating is controlled by the sample scan speed or high repetition rate systems where the period is controlled using modulation of the pulse train (for example using an acousto-optic modulator) have been suggested as a method of forming waveguide Bragg gratings.
Examples of methods of forming waveguide Bragg gratings using laser inscription have been described in WO 2007/134438, in which individual laser modified volumes in a transparent substrate with pre-determined distances between them in a transmission direction function as both gratings and waveguide structures. The technique described in WO 2007/134438 uses a single laser pass through the material to form the waveguide and grating structures in a sample, and the grating period is controlled by a scan speed of the sample through a focused pulse train. A similar technique is described in an article by Zhang and Herman at www.photonics.com/Article.aspx?AID=31911.
An alternative approach is described in Marshall et al (Opt.Letters, 31, 18, 2690) in which a double pass technique is used. A laser is scanned along a path through the material once to form a waveguide structure and is scanned along the same path a second time to modify the properties of the waveguide to form a grating structure.
However, the control over properties of waveguide or grating devices fabricated using known techniques of laser modification of bulk materials can be limited, and the range of waveguide or grating structures that can be produced in practice is also limited. That in turn limits the possibilities of using such techniques for producing practical sensor systems or for producing interrogators for such sensor systems.
It is an aim of the present invention to provide an improved or at least alternative method of forming optical devices.